1. Field of the Invention
This invention relates to a joined assembly of ceramic and metallic materials which maintains a high joining strength over a wide range of temperatures.
2. Description of the Related Art
Structural ceramics are used for a wide variety of applications because of their excellent properties including wear resistance and high-temperature strength. They, however, have also a number of drawbacks, including brittleness and low machinability, which limit the scope of their applicability. These drawbacks can be overcome if a ceramic material is used to make only that part of an object of which good wear resistance and high temperature strength are required, and is joined to a metallic material forming the rest thereof. The technique to be used for joining the ceramic and metallic materials is, therefore, of great importance. The methods which are conventionally employed for joining ceramic and metallic materials are divided into two main groups, i.e. mechanical joining methods such as shrink fitting, and chemical bonding methods such as diffusion bonding and active brazing.
Shrink fitting is a simple method, but has the drawbacks of calling for the use of materials having a high degree of machining accuracy, and yet failing to maintain the necessary torque at a high temperature causing the loosening of the assembly. If a larger shrinkage allowance is employed to overcome the latter drawback, an excessively high shrinking pressure acts radially upon the ceramic part and lowers its strength. Therefore, the joined assembly made by shrink fitting can withstand only a temperature up to the maximum temperature that allows the shrinking pressure to develop the necessary torque, or a temperature of about 300.degree. C. to 400.degree. C. The heat resistance of the joined assembly, however, depends on the combination of the materials forming it. It also depends on the method which is employed.
Shrinkage fitting with a filler metal is a modified form of shrink fitting and employs a hot brazing material as a filler for the clearance between the ceramic and metallic parts to be joined. As the brazing material is soft enough to serve as a stress relaxation material between the ceramic and metallic parts, the modified method enables the use of a larger shrinkage allowance than ordinary shrink fitting does. Therefore, it can make a joined assembly which is superior in heat resistance to the product of ordinary shrink fitting.
The size of the shrinkage allowance which is possible depends on the difference in coefficient of thermal expansion between the ceramic and metallic parts, the diameters of the parts to be joined, and the melting point of the brazing material. The use of a brazing material having a higher melting point enables a larger shrinkage allowance and the formation of a joined assembly with higher heat resistance. The use of a brazing material having too high a melting point, however, results in an excessively large shrinkage allowance. Furthermore, the higher the melting point of the brazing material, the harder it usually is, and the less effective it is for stress relaxation. This brings about a reduction in strength of the joint.
Silver brazing filler BAg8 (Japanese Industrial Standards) has a melting point of 780.degree. C. which is higher than that of any other brazing material that is usually employed for the shrinkage fitting with filler metals. A joined assembly formed by using this brazing filler can withstand a temperature up to about 500.degree. C., just below the softening point of the brazing filler. The use of a brazing filler having a still higher melting point, such as nickel brazing filler, makes it possible to obtain a joined assembly with still higher heat resistance, as the effective shrinking pressure is maintained up to a still higher temperature. In a low temperature range, however, excessive shrinkage pressure brings about an undesirable reduction in strength of the joined assembly.
The chemical bonding methods include: a method in which brazing is applied to a metallized surface; a brazing method which employs a brazing filler containing an active metal; and a diffusion bonding method in which solid materials are bonded together through solid diffusion under pressure.
No very high heat resistance can, however, be expected from any product of either of the brazing methods. This is because the maximum temperature that any brazed assembly can withstand is limited by the softening point of the brazing filler employed, and which is usually about 500.degree. C.
On the other hand, diffusion bonding can make a product with high heat resistance. For example, a product of silicon nitride and nickel obtained by the diffusion bonding can withstand a temperature up to about 800.degree. C.
In either event, however, the difference in coefficient of thermal expansion between the ceramic and metallic parts which are bonded together develops residual stress in the bonded assembly in the vicinity of ordinary temperature and thereby degrades its strength, though the interracial bonding strength between those parts may be satisfactory high. It is, therefore, known that a material having a coefficient of thermal expansion between those of the ceramic and metallic parts to be bonded should be interposed therebetween to relax stress. No such material can, however, remove the residual stress completely, but the strength of the bonded assembly as a whole is still lower than the interracial bonding strength between the ceramic and metallic parts.